<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>post方法</title>
</head>
<body>
<form autocomplete="off">
  姓名：<input type="text" id="username">
  <span id="uSpan"></span>
  <br>
  密码：<input type="password" id="password">
  <br>
  <input type="submit" value="注册">
</form>
</body>
      <script src="../js/jquery-3.3.1.min.js"></script>
      <script>
      $("#username").blur(function (){
        let username = $("#username").val();

        $.post(
            //请求的资源路径
                "userServlet",
                //请求参数
                "username="+username,
                //回调函数
                function (data){
                  $("#uSpan").html(data);
                },
                "text"
        );
      });
      </script>
</html>